Lathing

On brick or stone walls, lathing is usually attached to vertical furring strips, 1 in. thick by 2 in. wide, set at 12" or 16" centers. By this means there is a clear air space bet \ the plaster slabs and the wall, thus insuring a continually dry surface, which would otherwise be liable to dampness. In the case of walls in frame buildings, the lath is attached directly to the studs forming the framing of the walls. The ceiling lath may be nailed directly to the under edges of the joists, or attached to cross-furring, similar to that used on the walls, set at right angles to the joists and fixed at 12" centers. By the latter method better results are obtained, as the warping of the joists does not affect the lath. Lath may be either split or sawed; the former gives a better wall, as there are no cross-grained fibers to reduce the strength, while in the latter such fibers make the lath curl and warp from the absorption of moisture from the mortar; being cheaper, however, sawed lath is generally used. Lath made of pine, spruce, or hemlock, should be straight-grained, well seasoned, free from sap and rot, clear of shakes and large or loose knots, and, to prevent the subsequent discoloration of the plaster, it should be free from live knots and resinous pockets. The regular size of a lath strip is 1/4 in. X 1 1/2 in. X 4 ft., the length regulating the spacing of the furring strips, studs, and joists. Lath is nailed in place in parallel rows, the edges being kept a full 1/4 in. apart, to enable the soft plaster to be pressed through and form a key. The ends should not lap, but be butt-jointed and flush; continuous joints should not occur on one support, but the lathed surface should be divided into panels from 15 in. to 18 in. wide, and the joints be made to break on alternate supports, as shown by the panel abcd in Fig. 1, page 262, otherwise continuous cracks will be liable to disfigure the plaster. The lath is usually attached to joists and studs with cut or wire nails, about 1 1/8 in. in length and having large flat heads, one nail being used at each support. The nails should be galvanized, to prevent the moisture from attacking the iron and causing them to rust, as well as causing large yellow blotches to appear on the surface of the plaster.

There are several metallic substitutes for the wood lathing, such as wire netting, crimped, perforated, and expanded metal, all of which possess much merit, and are preferable to wood.

The carpenter fixes plaster grounds 2 in. wide, shown at e, e, to the masonry and framework, for the attachment of the joiner work. Plaster casings are also placed around window openings, and false jambs at doorways. For 3-coat work, the plaster grounds on brick or stone walls that are to be coated on the solid wall should be 5/8 in. thick, and those for lathed surfaces 7/8 in.; at no point should the surface of the solid wall or that of the lathing approach the face of the grounds nearer than the regulation thickness for the plaster, which will thus be about 5/8 in.